Reservations needed?

I will be traveling by train from Milano Centrale to Zermatt and then from Zermatt to Murren in September. I purchased the Bernese Oberland regional pass for my stay in the Oberland. Do I need reservations for the train tickets to and from Zermatt? Can I use my pass for part of my trip? If yes, how can I make a reservation using the pass for the discount from the US? Thanks!